Coiled tubing refers to a continuously string of steel pipe that is continuously milled and coiled onto a large take-up reel for transportation and handling. Coiled tubing is used in a wide range of oilfield services and operations. It can be run into and out of a well bore at a high rate, relative to straight, jointed pipe, and, unlike wire line, it can be pushed into the well bore. With diameters from 0.75 inches to more than 4 inches, coiled tubing can possess a yield and tensile strengths from 50,000 pounds-force per square inch (PSI) to more than 120,000 PSI, and has been manufactured in lengths greater than 30,000 feet. It has been used for drilling, logging, cleanouts, fracturing, cementing, fishing, completion and production related operations.
Coiled tubing injectors are machines for running coiled tubing and, in some cases, even jointed pipe, in and out of well bores. The name “coiled tubing injector” derives from the fact that, in preexisting well bores, the tubing may need to be forced or “injected” into the well through a sliding seal to overcome the pressure of fluid within the well, until the weight of the tubing in the well exceeds the force produced by the pressure acting against the cross-sectional area of the pipe. However, once the weight of the tubing overcomes the pressure, it must be held by the injector.
There are a number of different types and configurations of coiled tubing injectors capable of handling coiled tubing used in oilfield operations. The head of a typical coiled tubing injector used today is comprised of two continuous chains, each mounted on sets of spaced-apart sprockets, so that there is an extended length of chain between the sprockets. At least one of the chains is driven by a motor—typically hydraulic, though other types of motors can be used—connected to one or more of the sprockets. The chains are arranged so that the coiled tubing entering the injector is held between the chains by grippers mounted to each of the chains. The grippers are pressed against the outer diameter of the tubing thereby generating a frictional force parallel to the axis of the tubing. The frictional force is directly related to the normal force applied by the grippers.
A reel of coiled tubing, a coiled tubing injector, a control cabin and other equipment for operating the injector are mounted on a trailer and transported to a site as a “coiled tubing unit.” As coiled tubing is unspooled from the reel, or is spooled back onto the reel, it is guided into alignment with the chains of the coiled tubing injector by an arched tubing guide. When the coiled tubing injector is deployed at a well site, the arched tubing guide is connected to a frame of the coiled tubing injector so that it has a fixed relationship with the coiled tubing injector while the injector is being operated. Generally, the arched tubing guide is positioned or oriented so that the coiled tubing is threaded into the top of the head of the injector, between its rotating chains. Optionally, tubing can also be fed first into a straightener mounted to the frame, on top of the injector head. The straightener removes the bend in the tubing before it enters the injector head. (As used in this description, “coiled tubing injector” refers to the injector head alone or in combination with a frame, within which the head is placed, and/or a straightener, unless the context indicates otherwise.)
The reel must maintain tension on the coiled tubing to wind the tubing coiled on the reel and to keep it wound on the reel, as it is being unspooled or spooled. The arched tubing guide prevents the coiled tubing from kinking or otherwise being damaged by the tension the reel is applying to the tubing.
The arched tubing guide is typically attached to the frame of the coiled tubing injector in a manner that allows it to be attached or connected in different positions or orientations. For example, the best positioning or orientation may depend on the diameter of tubing being used and whether the tubing is being lowered or pushed into the well bore or pulled out of the well bore. When the pipe is coming off a reel, it has relatively more curve than when it is pulled from the well, which may affect how the arched tubing guide is fixed to the injector. Thus, although the arched tubing guide will have fixed relationship with the head of the coiled tubing injector during operation, this does not imply that the attachment of the arched tubing guide to the injector cannot allow for adjustment.
When being used, the coiled tubing injector with the arched tubing guide is positioned over the well head, high enough to accommodate one or more blow out preventers, a riser, and other equipment that might be connected to the wellhead through which the coiled tubing must pass before entering the well bore. The riser is made up from one or more sections of straight pipe that extends from the blow out preventers attached to the wellhead, and is used to accommodate elongated, rigid tools that are attached to the end of the coiled tubing before being lowered into the well bore. The coiled tubing injector is connected to the riser with a stripper, through which the coiled tubing is pushed or pulled. Because there is no derrick or platform, a mobile crane is driven to the site and is used to position and hold the injector in place. Alternatively, a temporary structure is erected above the wellhead, on which the coiled tubing injector is placed. In either case, the person operating the coiled tubing injector and the coiled tubing reel is located remotely from the coiled tubing injector, typically in a cabin on the trailer used to transport the reel and coiled tubing injector to the well site. Examples of risers, cranes, wellheads, reels, and coiled tubing units are shown in U.S. published patent application no. 20130175048, which is incorporated herein by reference for all purposes.
The injector controls the motion of the tubing while the reel provides tension in the tubing between the reel and the injector by being constantly back driven. The amount of tension imparted to the tube is determined by the amount of torque generated by the reel's drive and the radial distance of the working wrap of tubing to the reel's axis of rotation. Several thousands of pounds of tension between the reel and the injector are required to maintain a controlled wrapping of the tubing on and off the reel. With larger and stronger coiled tubing, more tension is required to control orderly spooling of the tubing on the reel and puts higher loads on all tubing handling equipment.
The correct amount reel torque for a specific situation is typically controlled by the operator. The operator's experience, intuition and visual observation of the wrapping of the tubing are the only guides for operation. This subjective method may lead to the tension rising too high and causing equipment failure, the most common of which is a collapse of the arched tubing guide. Examples of situations that can cause the tension to climb to dangerous levels are: 1) movement of the injector with the reel brake set, 2) failure to decrease the reel's torque output as the tubing is unwrapped (constant reel torque causes more tension at the base wrap compared to the top wrap), and 3) rapid deceleration of the injector when running out of hole (reel momentum can cause the tension to increase before reel slows to match injector speed).
Consequently, tubing handling components have been designed to ensure that the arched tubing guide or, if it is used, the strut that holds the tubing guide, has the lowest safety factor, meaning that as tubing tension increases the strut is the first component to fail. It fails by buckling. While no failure is desirable, such a failure is preferred to others as buckling of the strut provides visual feedback to the operator that an overload has occurred and is less likely to cause a catastrophic failure than other failure modes.